At present, the UoS store sells hoodies which are made up of 50/50 cotton and polyester. Cotton is extremely intense to grow, using 3,000 gallons of water for just one t-shirt, with the farming also using toxins and pesticides which seep into the water supply. Polyester is made using plastics, oil and fossil fuels, so is equally bad for the environment. Additionally, clothing screen printing uses plastic inks. 
 

To fit with the universities sustainability policies, I suggest you consider changing the material you and the ink with which the products are made. This website (https://blackwaterstudios.co.uk/) uses water based inks instead of plastic based ones, and prints onto both organic cotton and a blend of recycled materials, making it far more sustainable than your current store is. Though the price will naturally be higher, ordering in bulk would likely reduce this. An alternative option would be to ask someone local to source and print the hoodies for you, as this would lower the environmental impact of transporting the products. Of course, the items will cost more, but most people want a university hoodie/t-shirt etc. and expect it to be a little pricy. With SUSU having an extinction rebellion society, I'm sure many UoS students would be happy to pay slightly more for a lot less environmental impact.
 

I understand the store may not be your responsibility, or main priority, but clothing makes up for one of the biggest factors in climate change so should be factored into your sustainability policies.